This invention relates generally to a lock for a downhole apparatus and more particularly, but not by way of limitation, to a hydraulic lock for holding down an upper packer mounted on a packer mandrel connected to a lower packer disposed in an open well bore.
In fracturing a formation intersected by a well bore, it is known that two seals, referred to as packers, are set in the well at the upper and lower boundaries of the formation to be fractured. A pressurized fracturing fluid is then injected between the set packers through a tubing or pipe string on which the packers are carried into the hole. Such procedure can be used either when the well bore is lined with a casing or when the well bore is unlined (referred to herein as an open well bore, or the like). It is important that once the packers are set, they remain set (until specifically released) so that the fracturing fluid will be properly contained to achieve the desired fracturing and so that hazardous conditions are not thereby created. It is also important for the packers to remain set when a measuring tool, such as a precision caliper tool subassembly, is carried between them. Any movement of the packers that is communicated to a tool such as a caliper could produce false readings and seriously damage such a tool when it has its measurement arms extended.
When the well bore is lined with a casing or the like, known types of mechanical and hydraulic slips can be used to engage the casing so that upward movement of the top packer, such as in response to the pressure of the fracturing fluid exceeding the hydrostatic pressure existing above the top packer, is prevented. Preventing such upward movement can also sometimes be accomplished to some degree by "slacking off" the tubing or pipe string so that the weight of the string exerts a downward acting force on the packers.
When packers are to be set in open well bores, however, the aforementioned mechanical and hydraulic slips have not been helpful in anchoring the top packer against upward movement. Likewise, the use of "slacked-off" tubing has been inadequate in general because in deep wells where the slacked-off pipe weight would be sufficient to prevent upward movement, the weight has been known to create a force exceeding the loading characteristic of the packer, thereby damaging it. In shallower wells, the upward applied force exerted by the fracturing fluid can easily overcome the lesser pipe weight, thereby causing the top packer to become unseated.
The foregoing problem particularly pertains to upward movement of the top packer because the lower packer has the greater fracturing fluid pressure acting downwardly on it, and its downward movement is limited by an anchor pipe resting on the bottom of the hole or engaging the side wall of the bore. The interconnecting construction of conventional dual packers known to the art is such that this downward limitation is also applicable to the top packer so that it is only the upward movement of the upper packer which is of primary concern.
Although one can circumvent this problem by always casing or lining the well bore and by then using the known types of casing engaging locks, it is desirable to solve the problem in a manner whereby open hole packers can be securely set and locked in open well bores because this saves the time and expense of always having to case or line the well bore while still accomplishing reliable fracturing.
The foregoing exemplifies the particular need for a lock by which a top or upper packer can be locked relative to a bottom or lower packer to prevent upward movement of the upper packer in response to the fracturing fluid pressure exerted between the two packers when the packers are used in an open well bore. The satisfaction of this need, however, would also provide an improved lock useful in other types of downhole apparatus which require locking against relative movement between different parts of the apparatus.